Apparatus for preventing ice accretion

ABSTRACT

Aerofoils ( 22 ) of a gas turbine engine are provided with a coating ( 34 ) or filler ( 44 ) of viscoelastic material. As ice accretes on the aerofoils ( 22 ) during operation, the resulting aerodynamic stability imbalance induces vibration in the aerofoils ( 22 ). The viscoelastic material ( 34, 44 ) damps this vibration, and in so doing generates heat, which melts the ice away from the aerofoils ( 22 ). Heat-conducting members conduct the heat to regions of the component in which ice accretion is to be prevented. Alternative embodiments are described in which the pseudoelastic behaviour of a shape memory alloy ( 56 ), or eddy currents arising from the rotor blades&#39; rotation in an axisymmetric magnetic field, are used as sources of heat.

This invention relates to gas turbine engines, and more particularly tothose components that are prone to ice accretion in use.

Icing of the engine commonly occurs during flight through cloudscontaining supercooled water droplets or during ground operation infreezing fog. Ice can accumulate on the intake leading edge, the fanspinner, the fan blades and even further into the engine. Ice tends toform first on the leading edges of components, altering the airflow,reducing lift, increasing drag and adding weight. Relatively smallamounts of ice can have a disproportionate effect on aircraftperformance. Additionally, damage may result from ice breaking away andbeing ingested into the engine or hitting the acoustic material liningthe intake duct.

Anti-icing operations are conducted to prevent the bonding of snow andice to the component surfaces. Once bonded snow or ice has formed,de-icing operations are conducted to remove it. Conventional anti-icingand de-icing systems use hot air, bled from a compressor and ducted tothe areas of the engine requiring de-icing, or electrical heating of theparts concerned; sometimes a combination of the two is used. Other knownsystems have used ducted hot oil, microwaves or chemical de-icing means.

A disadvantage of known anti-icing and de-icing systems is that theyrequire additional hardware, in the form of bleeds and ducting for hotair, or heating elements and their associated control systems, which addweight and complexity to the engine. In addition, the need for warmedand pressurised air, or for electrical power, is detrimental to theoverall performance of the engine and reduces its efficiency.

Many components of gas turbine engines are subjected to vibration inuse. Not only the rotating compressor and turbine blades, but alsostatic components such as guide vanes and nacelles, are subjected tovibrations which reduce the fatigue lives of these components and whichcan lead to premature cracking if the amplitude of vibration issufficiently large.

It is known to use various methods to damp these vibrations, in whichthe vibrational energy is converted into another form of energy.Generally, heat is produced as a by-product of the damping process. Inthe design of gas turbine engines, such heat is conventionally regardedas undesirable, or its effects are ignored altogether.

There is increasing interest in forming fan blades for gas turbineengines from composite materials. This offers several advantages, amongwhich are weight saving and the ability to tailor the mechanicalproperties of a blade, for example in different directions. However, iceadhesion to composite materials is not well understood, and compositematerials are generally not good conductors of heat. Existing de-icingand anti-icing methods may not, therefore, be readily applicable tocomposite blades.

It is an object of this invention to provide an apparatus for preventingice accretion on a component, without the need for any additional systemor external energy supply. The nature of the apparatus makes itparticularly suitable for use with composite fan blades. Such anapparatus will therefore substantially overcome the disadvantages ofknown systems.

According to the invention, there is provided an apparatus forpreventing ice accretion on a component as set out in claim 1.

Embodiments of the invention will now be described, by way of example,with reference to the attached drawings in which

FIG. 1 is a general view of a gas turbine engine fan blade of knowntype, showing the accumulation of ice in the leading edge region;

FIG. 2 is a general view of a gas turbine engine fan blade according toa first aspect of the invention;

FIG. 3 is a partially cut away view of a gas turbine engine fan bladeaccording to a second aspect of the invention;

FIG. 4 is a schematic illustration of a suitable weave for the outerwrap of a fan blade, according to a third aspect of the invention;

FIG. 5 is a general view of the fan case of a gas turbine engine,including an apparatus according to a fourth aspect of the invention;

FIG. 6 is a general view of the fan case of a gas turbine engine,showing an alternative embodiment of the fourth aspect of the invention.

FIG. 1 shows a conventional fan blade 12, having a root portion 14 andan aerofoil portion 16. The fan blade 12 extends, in an axial directionindicated by the arrow A, between a leading edge 18 and a trailing edge20. In operation, ice 22 tends to accumulate near to the leading edge18. As outlined above, the presence of this ice 22 is detrimental to theproper operation of the gas turbine engine, and the release of ice fromthe blade surface can cause damage further downstream in the engine.

FIG. 2 shows a composite fan blade 22 according to a first aspect of theinvention, having a root portion 24 and an aerofoil portion 26. Theaxial direction is shown by arrow A, as in FIG. 1. A metal erosion strip32 protects the leading edge 28 of the aerofoil portion 26 from damageby foreign objects.

An outer layer 34, comprising adhesive paint and a painted erosionresistant coating, covers the remainder of the aerofoil portion 28 ofthe blade 22.

In operation, under icing conditions, ice begins to accumulate on theaerofoil surface. The additional mass of the ice will upset the balanceof the blade 22, promoting vibration. The presence of the ice alsochanges the aerodynamic shape of the blade 22, and the resultingaerodynamic instability is likely to lead to further vibration.

Any mechanisms linked to friction-type damping enable vibration to bechanged into heat. Because the painted materials forming the layer 34are viscoelastic their vibration will dissipate energy and tend to dampthe vibration, and simultaneously will generate heat within the layer34. This heat will either melt the ice, or at least will melt theinterface between the ice and the blade 22, releasing it from theaerofoil portion 28 of the blade 22. The balance and the aerodynamicshape of the blade 22 are thereby restored to their intended states, andthe sources of the vibration removed. The process is repeated as furtherice begins to accumulate on the aerofoil surface 28.

A second embodiment is shown in FIG. 3. Here, the viscoelastic material,a synthetic mix of epoxy and polyurethane, is provided as a filler 44for a hollow composite blade 42. Pins 46 of carbon fibre provide a heatconduction path from the viscoelastic filler 44 to the surface of theblade 42, and also add mechanical strength. The number and positions ofthese pins 46 may be arranged to optimise the heat transfer.

FIG. 4 shows part of the aerofoil surface of a composite fan blade 22,as shown in FIG. 2. In this third embodiment of the invention, The weave54 of the outer wrap of the blade 22 includes fibres 56 of a shapememory alloy (SMA).

A phenomenon known as pseudoelasticity occurs in SMAs when the alloy iscompletely composed of austenite (i.e. when the temperature is greaterthan A_(f), the temperature at which the austenite phase finishesforming). As an increasing force is applied to the SMA, the austenitebecomes transformed into martensite. This transformation occurs withoutany change in the temperature of the alloy. Once the loading isdecreased, the martensite begins to transform back to austenite (becausethe temperature of the alloy is still above A_(f)) and the SMA returnsto its original shape. This reverse transformation releases energy asheat (the energy that was originally put into the alloy by applying aforce to it). In the embodiment of FIG. 4, vibrations in the blade 22cause repeated loading and unloading of the SMA fibres 56, with aconsequent release of energy as heat on each unloading. The fibres 56therefore act as a source of heat.

The SMA fibres are preferably located towards the surface of the blade,because the vibration strain energy will be greatest further away fromthe neutral axis. To optimise the conversion of strain energy into heat,the SMA fibres may be concentrated in the areas of greatest vibration(for example, around the anti-nodes of the vibration modes). Theseregions of the blade may not be the same regions where ice tends toaccumulate, and so the heat may have to be transferred through theblade. This may be achieved using a network of heat-conducting wires orpins (as described in connection with FIG. 3). If heat is not requiredin the regions where the SMA fibres are located, the SMA fibres may beinsulated to maximise the heat available for transfer.

FIG. 5 shows a fan case 62 of a gas turbine engine. A single fan blade122 is shown—in a real engine there would be a circumferential array ofsuch blades. Each blade 122 has a root portion which locates in acentral hub 64. In operation, the fan blades 122 rotate about the engineaxis X-X. Each blade 122 has a metallic, or electrically conducting,tip.

Around the outside of the fan case 62 are electrical windings 66. Theseare used, as described in UK Patent application GB 0410778.5, togenerate a magnetic field around the fan assembly. The teaching of thispatent application is incorporated into this specification by reference.

As explained in GB 0410778.5, the electrical windings 66 generate anaxisymmetric magnetic field through which the fan blades (and, moreparticularly, the conducting tips of the fan blades) pass in theirrotation. Provided the tips of the fan blades do not deviate from theirdesign position and rotational path, any flux line of the axisymmetricfield will always pass through the same place in any blade, and so thereis no net force on any blade. Any deformation of a blade, or anydeviation in its path, will cause the flux lines to move relative to theblade and a restoring force will be set up. The vibration of the blades122 as a result of ice accretion, as described for previous embodiments,will cause such deformation and deviation, and consequently restoringforces will be set up. Heat will be generated within the blades 122 as aresult of these forces, and this heat will melt the ice as describedpreviously.

An alternative embodiment is shown in FIG. 6. As in FIG. 5, electricalwindings 66 are arranged around the outside of the casing 62. In thisembodiment, the electrical windings are arranged around an annularsupport 68. Actuators 70 permit the support 68 to be moved so that thesupport 68, and consequently also the electrical windings 66, are nolonger aligned with the engine axis X-X. This will tend to inducevibration in the blades, which will generate heat to melt the ice (asdescribed in the preceding paragraph) and may also shake off the icedirectly.

As a further alternative, the electrical windings 66 shown in FIGS. 5and 6 could be combined with fan blades having viscoelastic coatings orfillers (as shown in FIGS. 2 and 3) or having SMA inserts (as shown inFIG. 4) and the vibrations induced by the magnetic field would then leadto heating of the viscoelastic material or SMA, as explained above.

Other modifications are possible to the embodiments described, withoutdeparting from the scope of the invention.

In the embodiment of FIG. 2, for example, the viscoelastic material maybe covered by a face sheet of a material with relatively high thermalconductivity. This will improve the heat transfer from the viscoelasticmaterial into the ice layer. Alternatively, discrete strips of such amaterial may be overlaid on the viscoelastic material, in any desiredpattern. This latter solution may provide less constraint to theviscoelastic material, and thus avoid any impairment of its performance.

In the embodiment of FIG. 3, other materials may be used for the filler44. Instead of a synthetic mix of epoxy and polyurethane, either epoxyor polyurethane may be used alone. Polyethylene may also be used.Various materials may be added to these basic constituents, to reducethe density of the filler or to increase its toughness or stiffness.Examples of suitable additives are: microspheres (e.g. of glass,ceramic, metallic, polymer, or metallic coated ceramic or glass); solidspheres (e.g. of polystyrene or rubber); fibres (e.g. of aramid, silk,metal or carbon).

Although the invention described is particularly suitable for use withcomposite fan blades of gas turbine engines, it will be understood thatthe principles may be applied to other components, and in other types ofmachinery, with equally beneficial effects. For example, the inventioncould be applied to propellers, unducted fans, static vanes, nacelles,splitter fairings or CIAM tip treatments.

1. An apparatus for preventing ice accretion on a component subjected inuse to vibration, in which in use heat is generated by damping of thevibration, characterised in that the component includes at least oneheat-conducting member that in use conducts the heat to a region of thecomponent in which ice accretion is to be prevented.
 2. An apparatus asin claim 1, in which the damping is provided by a coating ofviscoelastic material on at least part of the component.
 3. An apparatusas in claim 2, in which the heat-conducting member is a face sheetcovering the viscoelastic material.
 4. An apparatus as claimed in claim2, in which the heat-conducting members are strips provided on at leastpart of the viscoelastic material.
 5. An apparatus as in claim 1, inwhich the damping is provided by a filling of viscoelastic material inat least part of the component.
 6. An apparatus as in claim 5, in whichthe heat-conducting members conduct the heat from the filling to thesurface of the component.
 7. An apparatus as in claim 6, in which theheat-conducting members are pins, rivets or stitches.
 8. An apparatus asin claim 2, in which the viscoelastic material comprises one or more ofepoxy, polyurethane and polyethylene.
 9. An apparatus as in claim 2, inwhich the viscoelastic material includes one or more additives selectedfrom: microspheres, coated microspheres, solid spheres, fibres.
 10. Anapparatus as in claim 2, in which the glass transition temperature ofthe viscoelastic material is optimised to give maximum heating at theoperating temperature of the component where icing is most likely tooccur.
 11. An apparatus as in claim 1, in which the damping is providedby shape memory alloy elements incorporated in the component.
 12. Anapparatus as in claim 11, in which the elements are wires incorporatedin the weave of a fibre-reinforced composite structure.
 13. An apparatusas in claim 11, in which the heat-conducting member is a face sheetcovering at least some of the elements.
 14. An apparatus as claimed inclaim 11, in which the heat-conducting members are strips contacting atleast some of the elements.
 15. An apparatus as in claim 11, in whichthe heat-conducting members conduct the heat from the elements to thesurface of the component.
 16. An apparatus as in claim 11, in which theheat-conducting members are pins, rivets or stitches.
 17. An apparatusas in claim 2, in which the component is a component of a gas turbineengine.
 18. An apparatus as in claim 17, in which the component is a fanblade, propeller, unducted fan blade, nacelle or splitter fairing. 19.An apparatus as in claim 1, in which the damping is provided by amagnetic field generated around the component.
 20. An apparatus as inclaim 19, in which the magnetic field can be distorted to inducevibration in the component to aid in the shedding of ice.
 21. Anapparatus as in claim 20, in which the magnetic field is generated byelectrical windings surrounding the component, and the magnetic field isdistorted by changing the alignment of the electrical windings relativeto the component.
 22. An apparatus as in claim 19, in which thecomponent is a component of a gas turbine engine.
 23. An apparatus as inclaim 22, in which the component is a fan blade.
 24. An apparatus as inclaim 22, in which the magnetic field is generated by electricalwindings arranged around a casing of a gas turbine engine.